1. Field of the Invention
This invention relates to rectangular wave guide elbows such as H-elbows which are bent across the narrow dimension of the wave guide and which have outer corners which are symmetrically angled with a conductive flattening plane.
2. Description of the Prior Art
Such elbows are known and are described in the publication "Taschenbuch der Hochfrequenztechnik" by H. Meinke and F. W. Gundlach, Springer Verlag, 2nd Edition, 1962, at pages 401 and 402. Such elbows are employed in various microwave circuits which utilize rectangular wave guides. By using angled wave guides, a more compact structure is achieved relative to the comparable low refraction circular arc elbows, particularly when used with wave guide diplexers of different types as, for example, frequency diplexers, polarization filters, wave mode filters, etc. Generally, wave guides with a rectangular cross-section having side ratio a:b=2:1 are most frequently used. Such wave guides can be used in the relative frequency range for the maximum band width of f.sub.o :f.sub.u =2:1 for the TE.sub.10 wave. The above referenced publication discloses that the reflection of an H-elbow can be reduced if the external corner of the elbow is symmetrically flattened with a conductive plane and this publication teaches that corner flattenings or smoothings of various degrees can be utilized and that there is an optimum cathetus length x.sub.Ho as illustrated in FIG. 1b which is the distance from the apex of the untruncated elbow to the junction of the smoothing conductive plane with the narrow side of the wave guide as illustrated. This dimension has a ratio relative to the wide dimension of the wave guide of 0.64 for optimum conditions as described in the previously referenced publication. For this cathetus dimension ratio of 0.64 the reflection of an H-elbow will remain under r=16.7% in the frequency range of a rectangular wave guide over the frequency range of 1.25fcTE.sub.10 through 1.9fcTE.sub.10. Only in smaller frequency bands within this range can smaller reflections be achieved and for this purpose the cathetus dimension can be changed somewhat relative to x.sub.Ho according to the position of the partial frequency band within the full wave guide frequency band range.
FIG. 1a illustrates the respective conductive ripple s for H-elbows for wave guides wherein the H-elbow is bent at an angle of 90.degree. and illustrates curves for a few selected ratios of the dimension x.sub.H /a for the corner flattening or smoothing as illustrated in the dimensions in FIG. 1b. Proceeding from the ripple s which exists for an H-elbow measured over an entire wave guide pass band and the lower curve in FIG. 1a designated "0" is for a curve where the exterior corner of the H-elbow is not flattened and x.sub.H /a=0. The curve designated 0 in FIG. 1a illustrates the variation of reflection and ripple with frequency relative to the angle bisecting cross-section plane in the wave guide extending from the apex of the bend to the internal corner bend. The disruptive reflection has a capacitive phase over the entire wave guide pass band because of the expanded broad side of the rectangular wave guide in the bend area. Since the wave guide does not have corner flattening, the rectangular wave guide will have a broad side that is locally expanded to different degrees and has a correspondingly greatly reduced TE.sub.20 critical frequency (.lambda.cTE.sub.20 =a). Also, a very noticeable maximum reflection occurs in the upper part of the wave guide pass band and the cause of this reflection is an undesired TE.sub.20 resonance in the area of the bend.
A TE.sub.201 resonance which is induced across the magnetic TE.sub.10 field at the bend is formed in the bend area under the higher TE.sub.20 critical frequency of the straight wave guide.
It is known in the previously referenced publication "Taschenbuch der Hochfrequenztechnik" to symmetrically flatten the H-elbow as illustrated in FIG. 1b at the outer corner with a conductive plane and the size of the flattening or smoothing plane is determined by means of the cathetus dimensions x.sub.H. This flattening functions to counteract the broad side expansion of the non-flattened H-elbow and thus reduces the capacitive effect and increases the inner TE.sub.20 resonance at the higher frequencies.
With the optimum corner flattening x.sub.Ho /a=0.64 as disclosed in the publication "Taschenbuch der Hochfrequenztechnik" results in compensation of the ripple s in the center of the wave guide pass band as shown in FIG. 1a by the curve which is designated 0.64. However, the ripple s increases in the lower portion of the band pass up to a value of s.sub.u =1.4 which corresponds to a reflection factor of r.sub.u =16.7% and at the upper frequency band limit up to s.sub.o =1.23 which corresponds to a reflection factor of r.sub.o =10.3%. These values of reflection cause a significant disruption in many cases where the wave guide is used and thus the method and apparatus for compensating the ripple factor by flattening the corner does not result in sufficient reduction of the reflection factor.
The publication "Taschenbuch der Hochfrequenztechnik" teaches how the values x.sub.H /a of the corner flattening is reduced or increased so as to shift the matched point above or below the center frequency of the pass band. However, undesirable increase of the reflection in the lower frequency band or, alternatively, in the upper part of the frequency band occur. As shown in FIG. 1a, the matched point in the limiting case can be carried down to TE.sub.10 critical frequency of the wave guide if the value of the ratio of x.sub.H /a=0.74.